In a wireless communication system, a transmitting end and a receiving end generally use multiple antennas for transmission and reception to reach a higher rate. One principle of the multiple-input-multiple-output (MIMO) technology is to implement multi-layer transmission which matches channel characteristics by using some channel characteristics, thereby effectively improving system performance. Significant performance improvement is obtained without increasing a bandwidth and power, so that the MIMO technology is very promising and has wide applications in the current system. For example, the Long Term Evolution (LTE) system and the Long Term Evolution-Advanced (LTE-A) system have multiple transmission modes for a multi-antenna technology, i.e., a transmission mode 2 to a transmission mode 10. The multi-antenna technology involves many concepts and techniques. To help understand and describe the content of the present disclosure, some concepts of key techniques will be introduced below.
Generally, channel state information (CSI) is fed back in two manners, that is, periodic feedback and aperiodic feedback. For example, in the LTE/LTE-A system, a physical uplink control channel (PUCCH) is adopted to perform the periodic feedback and a physical uplink shared channel (PUSCH) is adopted to perform the aperiodic feedback. A terminal mainly feeds back the CSI in two manners. A base station may configure the terminal to measure and quantize the channel information and perform the periodic feedback on the quantized channel state information (CSI) through the PUCCH. The CSI includes a rank indicator (RI)/precoding matrix indicator (PMI)/channel quality indication (CQI). When necessary, the base station may also aperiodically trigger the terminal to report the CSI (including the RI/PMI/CQI) mainly through the PUSCH, implement the feedback with a high real-time feature and the higher CSI quantization accuracy.
The basic principle of codebook-based channel information quantization feedback is briefly described as follows. Assuming that a limited feedback channel capacity is B bps/Hz, the number of available codewords is N=2B. The feature vector space of a channel matrix is quantized to form a codebook space ={F1, F2 . . . FN}. The transmitting end and the receiving end jointly store or generate the codebook in real time (which is the same at the transmitting end and the receiving end). According to the channel matrix H obtained by the receiving end, the receiving end selects a codeword {circumflex over (F)}, which best matches the channel, from  according to a certain rule and feeds back a serial number i of the codeword (that is, the PMI) to the transmitting end. The transmitting end finds the corresponding precoding codeword {circumflex over (F)} according to the serial number i and obtains the channel information, where {circumflex over (F)} represents the feature vector information of the channel.
The principle for constructing the codeword in the LTE system is introduced as follows. The codebook of the LTE is also evolving with the evolution of a standard release. In the release 8 and the release 9, the 4-antenna codebook and the 2-antenna codebook both are in a form of single codeword. There is only one PMI, the value of which is represented as i=1, . . . , N11, where N11 is the number of codewords. For an 8-antenna codebook in the release 10 and a 4-antenna codebook in the release 12, a dual codebook feedback form is used. That is, the codeword may be written as W=W1*W2, where W1 is a long-term feedback codebook called a first codebook and W2 represents a short-term feedback codebook called a second codebook. W2 is adopted to select one from M1 alternative beams in codewords of W1 and select a beam selection polarization phase, co-phasing, for each polarization direction at the same data layer. Each codeword in W2 is quantized and fed back by PMI2 whose value is i2=1, M1, where M1 is the number of W2. For details, reference may be made to the LTE release 10.
The codewords in releases before the release 12 are targeted for 1D (one-dimensional) antenna arrays and belong to 1D codewords. In a design of the codebook in the release 13, the dimension of the codebook becomes larger due to the use of more antennas. A topology of antennas is also generally planar, that is, 2D (two-dimensional) codewords are designed for antennas with two dimensions. Therefore, each beam in the first codebook W1 has a two-dimensional form vm⊗un, where vm and un are respectively discrete Fourier transform (DFT) vectors in the first dimension and the second dimension, vm⊗un represent a kronecker product of vm and un, where m=1, 2, . . . , B1, and n=1, 2, . . . , B2. The number of ports (in the present disclosure, the ports include an antenna/port/transport unit/element/array element and another apparatus capable of sending signals) is N1 and the number of ports in the second dimension is N2. Oversampling at O1 times is performed in the DFT corresponding to the ports in the first dimension and oversampling at O2 times is performed in the DFT corresponding to the ports in the second dimension. The number of DFT vectors of antennas in the first dimension or the second dimension is a multiple of an oversampling factor of the number of ports and thus B1=N1*O1 and B2=N2*O2, where O1 is an oversampling factor in the first dimension and O2 is an oversampling factor in the second dimension. A codebook in the first dimension of the first codebook is represented by PMI11 whose value is i11=1, . . . , N11, and a codebook in the second dimension of the first codebook is represented by PMI12 whose value is i12=1, . . . , N12. For each index of PMI11 and PMI12, the number M1 of W2 codewords exists and each W2 codeword is used for selecting a two-dimensional beam vm⊗un from W1 and the co-phasing of different polarization directions, and the corresponding codeword index is PMI2 represented by i2=1, . . . , M1.
Without loss of generality, the codeword of the number N11=1 of ports in the first dimension or the number N12=1 of ports in the second dimension is the 1D codeword; and the codeword of the number N11>1 of ports in the first dimension or the number N12>1 of ports in the second dimension is the 2D codeword. The 1D codeword in a single codeword structure is represented by PMI or i, the 1D codeword in a dual codeword structure is jointly represented by PMI1 and PMI2 and the index is jointly represented by i1 and i2. the 2D codeword is jointly represented by three codebook indexes, PMI11, PMI12 and PMI2, or jointly represented by indexes i11, i12 and i2.
In the current system, the precoding matrix or the configured beam is fed back based on information of a strongest path in the channel and the information of other paths in the channel is ignored. In this way, the information which is fed back or configured cannot match the channel well, and the system performance is thus affected. Therefore, the LTE-A system introduces a codebook based on linear weighting and combination of information of multiple paths, thereby greatly improving feedback accuracy and the system performance.
Although the linear codebook based on combination of multiple paths can better match the channel, since the amplitude weighting coefficient and phase weighting coefficient of every piece of path information need to be fed back or configured in the combination process, the system will have a huge overhead.